1. Field of the Invention
The present invention relates to an optical semiconductor device and a method for fabricating the optical semiconductor device, more specifically to an optical semiconductor device including quantum dots and a method for fabricating the same.
2. Description of the Related Art
Semiconductor optical amplifiers and semiconductor lasers are much noted in the field of optical communication, etc. for their small sizes and small electric power consumption.
A conventional optical semiconductor device will be explained with reference to FIG. 12. FIG. 12 is a sectional view of the conventional optical semiconductor device.
As shown in FIG. 12, a clad layer 112 of n type InP is formed on a semiconductor substrate 110 of n type InP.
A quantum well active layer 124 is formed on the clad layer 112.
A clad layer 136 is formed on the quantum well active layer 124. The clad layer 136, the bulk active layer 124 and the clad layer 112 generally form a mesa. Thus, an optical waveguide layer 138 in mesa is formed.
A current restricting layer 118 formed of a p type InP layer 118a and an n type InP layer 118b is formed on both sides of the optical waveguide layer 138.
A cap layer 140 is formed of n type InP on the optical waveguide layer 138 and the current restricting layer 118.
An AR (Anti-Reflection) coated film (not shown) is formed on both end surfaces of the mesa-optical waveguide layer 138. Thus, the conventional semiconductor optical amplifier is constituted.
However, the above-described conventional optical semiconductor device has a narrow gain bandwidth, and cannot amplify at once WDM (Wavelength Division Multiplexing) signals in a wide band.
Then, it is proposed to use quantum dots as the active layer. FIG. 13 is a conceptual view of the proposed optical semiconductor device.
As shown in FIG. 13, quantum dots 102 are formed in an upper part of a semiconductor substrate 100.
Quantum dots are thus used as the active layer, whereby an optical semiconductor device having a wide gain bandwidth can be provided.
However, the proposed optical semiconductor device shown in FIG. 13 has high polarization dependency. In order to make the proposed optical semiconductor device practically usable, it is necessary to reduce the polarization dependency.